Method of producing oriented graphite



Oct. 10, 1967 L. PODOLSKY 3,346,416

METHOD OF PRODUCING ORIENTED GRAPHITE Original Filed May 28, 1962 IN VENTOR' 45a P000490 ATTOPl/EY United States Patent 3,346,416 METHOD OF PRODUCING ORIENTED GRAPHITE Leon Podolsky, Pittsfield, Mass., assignor to Drexel Institute of Technology, a corporation of Pennsylvania Original application May 28, 1962, Ser. No. 198,286, now

Patent N 0. 3,142,158. Divided and this application Mar.

26, 1964, Ser. No. 355,001

3 Claims. (Cl. 117226) This application is a division of copending application Ser. No. 198,286, filed May 28, 1962, now US. Patent No. 3,142,158.

This invention relates to the deposition of graphite films, and more particularly to the production of oriented graphite.

The term oriented graphite is used herein to mean graphite, the crystals of which are all, or substantially all, oriented with their 0 axes in a single direction.

It is a general object of the present invention to provide a novel method of producing oriented graphite.

It is another object of the invention to provide a method for coating the surface of a body with a film of oriented graphite.

It is a more specific object of the invention to provide a method for depositing oriented graphite on a metal tube.

Oriented graphite produced by the method of the present invention has many potential uses. For the sake of illustration, and to render the invention completely understandable, one purpose for which the present method has been found particularly advantageous will be referred to hereinafter. It is the making of a thermoelectric cooling device such as that described in the above-mentioned copending application.

In that application, a cooling device is described in which passage of an electric current through a thermocouple is caused to produce the desired effect. It is of course well known that when two dissimilar electrically conductive materials are placed in contact thereby forming a thermocouple, and an electric current is passed through them in a certain direction, a reduction in temperature takes place at the junction between the two materials.

In the device mentioned above, a metal tube serves as the self-sustaining member of a thermocouple, and the tube carries on its outer surface an electrically conductive graphite film. The tube and film form a thermocouple which, when connected in series to a source of electric current, becomes cooled. If the tube is employed as part of a fluid circuit, the fluid moving through the circuit is cooled as it flows through the tube.

As discussed in applicants copending application Ser. No. 195,681, filed May 18, 1962, now Patent No. 3,330,- 703, in order that the graphite have a maximum figure of merit the crystals of the graphite film must have a particular orientation with respect to the flow of electric current through the graphite. As mentioned above, the present invention provides a convenient and thoroughly practical method for accomplishing the graphite crystal orientation.

Other objects and advantages of the invention will be apparent from the following detailed description in which reference is made to the accompanying drawings.

In the drawings:

FIG. 1 is a perspective view of a metal tube;

FIG. 2 is a cross-sectional view of a furnace for deposit ing an oriented graphite film on tubes such as the one shown in FIG. 1; and

FIG. 3 is a view similar to FIG. 1 showing the tube carrying a graphite film on its outer surface.

The present invention is illustrated in connection with 3,346,416 Patented Oct. 10, 1967 a tubular thermoelectric cooling device. However, it is to be understood that the usefulness of the invention is not limited to cooling devices or to tubular surfaces.

As is well known, the performance characteristics of thermoelectric devices can be conveniently rated by a value known as the figure of merit. This parameter is proportional to the square of the thermoelectric E.M.F., and inversely proportional to resistivity and thermal conductivity of the couple, all measured in the direction of current flow through the junction of the thermocouple, and the figure of merit is advantageously as high as possible. Graphite does not exhibit a high enough figure of merit for practical purposes unless deposited on the tube in such a way that its crystals have a particular orientation mentioned below.

The metal tube 10 may be provided with a graphite coating 11 in a deposition furnace 12 (FIG. 2). Deposition furnaces are well known and may include a cylindrical shell 13 into which is introduced a hydrocarbon gas, such as methane, and which is heated, as by electric means, to a temperature (of the order of 2100 F.) capable of burning and cracking the hydrocarbon gas. According to the present invention the shell 13 of the furnace is girdled by an outer tubular electrode 14. A metal rod 15 serving as an inner electrode extends longitudinally through the furnace. The rod is appropriately formed to carry the metal tube or tubes 10 to be coated. Throughout the cracking operation which causes crystalline graphite to be deposited on the relatively cool surfaces of the tube 10, a high direct current potential difference is applied between the electrodes 14 and 15 producing an intense electrostatic field within the furnace. An electrostatic field of several thousand volts per inch is usually employed. The direction of this field will obviously be perpendicular to every point on the outer surface of each tube 10, and the effect of the field will be to cause the graphite crystals to be deposited with their c axes in the direction of the field, i.e., the c axes of the deposited crystals Will be perpendicular to the surface of the tube upon which they are deposited.

A graphite crystal exhibits along its c axis its highest thermoelectric and its lowest thermoconductivity (both favorable factors as far as figure of merit is concerned). However, the resistivity along this axis is highest, a factor which normally tends to reduce the figure of merit. Nevertheless, when the graphite is employed in the form of a thin film or coating, and the direction of current fiow is transversely through the film, the resistivity is negligible even though the current flow is along the c axis.

In the drawings, the thickness of the film has been greatly exaggerated for clarity. In practice, the film is ordinarily less than twenty microns in thickness.

The tube 10 and film 11 are, of course, a thermocouple, and if the film is connected to one terminal of an electrical source and the tube to the other terminal, the tube and film experience a reduction in temperature.

The invention has been shown and described in terms of a method for coating a metal tube with a film of graphite, the crystals of which are oriented with their 0 axes perpendicular to the surface of the tube. However, it is obvious that the invention has much broader significance with respect to production of oriented graphite in general. It is understood, therefore, that the invention is not limited to any specific form except insofar as such limitations appear in the appended claims.

What is claimed is:

1. A method of producing oriented graphite comprising the steps of providing a furnace, providing a surface within the furnace upon which the graphite may be deposited, heating the furnace to a temperature of the of said heating step creating an electrostatic field at said surface,-the direction of said fieldrbeing the same as the desired direction of the c axes'of the graphite crystals deposited on said surface as a result of the cracking 'operation. 7

2. A method of coating the surface of a body With a film of oriented graphite comprising the steps of providing a furnace, heating the furnace to a temperature of the order of 2100 F. suflicient to crack a hydrocarbon gas, introducing a hydrocarbon gas into the furnace, placing the body to be coated into the furnace, and independently of said heating step creating an electrostatic field at and perpendicular to the surface of the body to be coated whereby graphite crystalsv deposited on said surface as a result of the cracking operation Will be oriented with their c axes perpendicular to said surface.

3. A method of depositing oriented graphite on a metal '2100" F. sufficient to crack a hydrocarbon gas so as to deposite graphite on said tube, and independently of said 7 heating step creating an electrostatic field between an electrode within said tube and an electrode surrounding said 7 tube to cause the graphite to be deposited on said tube with the c axes of its crystals perpendicular to the outer surface of said tube.

References Cited UNITED STATES PATENTS 2,788,296 4/1957 Louis 117-226 X 2,972,555 2/1961 Deutscher 117-95 X 3,081,485 3/ 1963 Steigerwald 117-17 3,138,435 6/ 1964 Diefendorf 117226 X r 3,167,449 1/1965 Spacil 117226 X 3,172,774- 3/1965 Diefendorf 117226 X Diefendorf 117226 X ALFRED L. LEAVITT, Primary Examiner. r

A. GOLIAN, Assistant Examiner. 

1. A METHOD OF PRODUCING ORIENTED GRAPHITE COMPRISING THE STEPS OF PROVIDING A FURNACE, PROVIDING A SURFACE WITHIN THE FURNACE UPON WHICH THE GRAPHITE MAY BE DEPOSITED, HEATING THE FURNACE TO A TEMPERATURE OF THE ORDER OF 2100*F. SUFFICIENT TO CRACK A HYDROCARBON GAS, INTRODUCING SUCH A GAS INTO THE FURNACE, AND INDEPENDENTLY OF SAID HEATING STEP CREATING AN ELECTROSTATIC FIELD AT SAID SURFACE, THE DIRECTION OF SAID FIELD BEING THE SAME AS THE DESIRED DIRECTION OF THE "C" AXES OF THE GRAPHITE CRYSTALS, 